Good service usually brings a customary 15% to 20% of the total bill before any discounts or coupons.  Always tip on the original amount before the discounts or coupons are applied.  

If the service was excellent, you should leave a larger than standard tip.  Something over 20% is appropriate.  The exact amount is up to you.

In some cases a restaurant will automatically add a tip, usually 15% to 18% of the check.  Normally this applies to large parties.  More restaurants seem to be including tips to all bills as a standard policy.  Always check your check to see if an additional amount is warranted.